Analysis of the disc components of our galaxy via kinematic and spectroscopic procedures

Abstract

We used the spectroscopic and astrometric data provided from the GALactic Archaeology with HERMES (GALAH) Data Release (DR2) and Gaia DR2, respectively, for a large sample of stars to investigate the behaviour of the [α/Fe] abundances via two procedures, that is, kinematically and spectroscopically. With the kinematical procedure, we investigated the distribution of the [α/Fe] abundances into the high-/low-probability thin disc, and high-/low-probability thick-disc populations in terms of total space velocity, [Fe/H] abundance, and age. The high-probability thin-disc stars dominate in all sub-intervals of [α/Fe], including the rich ones: [α/Fe] > 0.3 dex, where the high-probability thick-disc stars are expected to dominate. This result can be explained by the limiting apparent magnitude of the GALAH DR2 (V < 14 mag) and intermediate galactic latitude of the star sample. Stars in the four populations share equivalent [α/Fe] and [Fe/H] abundances, total space velocities, and ages. Hence, none of these parameters can be used alone for separation of a sample of stars into different populations. High-probability thin-disc stars with abundance -1.3 < [Fe/H] ≤ -0.5 dex and age 9 < τ ≤ 13 Gyr are assumed to have different birth places relative to the metal-rich and younger ones. With the spectroscopic procedure, we separated the sample stars into α-rich and α-poor categories by means of their ages as well as their [α/Fe] and [Fe/H] abundances. Stars older than 8 Gyr are richer in [α/Fe] than the younger ones. We could estimate the abundance [α/Fe] = 0.14 dex as the boundary separating the α-rich and α-poor sub-samples in the [α/Fe] [Fe/H] plane.